This invention relates to gradient coil assemblies for use in magnetic resonance imaging and spectroscopy (MRIS).
Typically, an MRI machine will include at least three independent electrical windings, each one typically used to encode one Cartesian dimension (X, Y and Z). Thus, typically there will be X, Y and Z coils in a gradient coil assembly of an MRI machine.
The windings may carry up to several hundred amperes and are typically energised and de-energised over periods as short as 100 microseconds. Large voltages (typically up to 2 kV) must be applied to the windings to achieve such switching. The voltages are applied over a few microseconds. Separate windings are energised and de-energised independently, but on occasions more than one winding may be energised or de-energised simultaneously. Such instances increase yet further the potential differences present in the coil structure.
Gradient coil assemblies are typically impregnated with epoxy resins after assembly to ensure good electrical and mechanical integrity. It is a known property of such resin systems that above a certain potential difference threshold, a phenomenon known as “partial discharge” occurs in regions of high electrical stress. This phenomenon is the result of microscopic charge redistribution around the inner surfaces of voids in the dielectric. Such discharges create broadband electrical interference that is deleterious to the sensitive radio frequency detection systems used in MRIS.
It is generally agreed that partial discharge inception voltage (PDIV) occurs at lower voltage levels if there are either bubbles of air in the insulation system or there are any sharp points present on the metal coil portions making up the windings in the gradient coil assembly.
The X, Y and Z coils that make up a typical gradient coil may be manufactured from plates of copper or another suitable metal having cut patterns to form current paths, or they may be wound either from solid or hollow metal conductors. Once the paths are formed, the resulting coil is mechanically consolidated so that it can be handled without the metal turns unraveling or changing shape in an uncontrolled way. The coils are often consolidated and held in place by some kind of non-conducting backing/substrate.
In one method, a coil has its patterns formed and then to consolidate the coil turns, a composite backing is bonded to the coil using epoxy or some other resin and a hot press. Once consolidated, the coil may be formed into a non-planar shape as required without the current paths moving in an undesirable way. Once the gradient coils have been formed, they are assembled into a gradient coil assembly. The whole assembly is usually vacuum impregnated with an epoxy or some other resin and then cured for a period of time to consolidate the entire assembly.
Epoxy resins, glass cloths, and other insulating materials generally have high dielectric strengths and relatively high relative dielectric constants between about 2 and 6. In the absence of defects, they can withstand the voltage levels typically employed in MRIS. However, if there are voids in the insulation, the large difference in relative dielectric constants between the material and the void causes enhancement of electric field in the void, and PDIV may occur at relatively low voltage levels (eg 1 kV).
Two major causes of low PDIV are air bubbles and sharp points or burrs on the metal coils.
Where there are air bubbles, the electric field in the air bubble may be much higher than in the material surrounding the air bubble. Electrical discharge occurs in air at a field strength of about 3 kV/mm. This type of field strength can occur in an air bubble in the gradient coil assembly in an MRIS machine.
Sharp points also lower PDIV which, generally speaking, is due to the field being concentrated at the sharp point. If a burr is present in the region of an air bubble in the insulating material in a gradient coil then the field enhancement created by the burr is liable to cause partial discharge.
As mentioned earlier, to build gradient coil assemblies it is usual to consolidate individual coils/windings and to build those coils into a complete coil assembly. Thus it is likely that the coils will be backed by partially cured epoxy resin impregnated materials (known as B-stage materials). B-stage materials have air pockets in them that cannot, in practical terms, be completely eliminated.
Further, the windings in the coils will have sharp points on them. It is not reasonable to expect to machine and consolidate coils and have no bubbles or burrs left at the end of the process.
However, in a region where B-stage material is absent it can be reasonable to expect that a vacuum impregnation process can yield a void free region.
Thus, it has been realised that if it is possible to construct a gradient coil assembly where B-stage materials and burrs are restricted to regions of low electric field strength, it should be possible to construct gradient coil assemblies having higher PDIV.